1. Field of the Invention
The present invention relates to a PLL oscillation circuit. More specifically, the present invention relates to: a PLL oscillation circuit that can reduce variability of a modulation sensitivity of a voltage-controlled oscillator (VCO) and obtain a desired output amplitude quickly with high precision; and a polar transmission circuit and a communication device, both of which utilize the PLL oscillation circuit.
2. Description of the Background Art
When utilizing a voltage-controlled oscillator (VCO) in a direct modulation method, a linear modulation sensitivity of the VCO is required in order to conduct a linear frequency modulation. However, a modulation sensitivity of the VCO is generally non-linear.
Described in the following is a reason for why the modulation sensitivity of the VCO is non-linear. FIG. 10 is a figure showing one example of a circuit configuration of a VCO. In the example shown in FIG. 10, a differential cross-coupled LC oscillator is used as the VCO. FIG. 11 is a figure showing one example of C-V characteristics depending on an output amplitude Vo (Vo=Vop−Von) of the VCO. Shown here are the C-V characteristics of the VCO when the output amplitude Vo of the VCO is 0.5 V and 1.5 V. As shown in FIG. 11, a fluctuation in an average capacitance value of a MOS varactor due to a fluctuation of the output amplitude Vo of the VCO is a reason for the variability of the modulation sensitivity of the VCO.
Therefore, the input-output characteristic of the VCO becomes non-linear as shown in FIG. 12. FIG. 12 is a figure that describes the nonlinearity of the VCO. In FIG. 12, the horizontal axis represents the input voltage (Vtune) of the VCO and the vertical axis represents the output frequency (fout) of the VCO. In addition, a dotted line represents an ideal (linear) input-output characteristic of the VCO, and a solid line represents an actual (non-linear) input-output characteristic of the VCO. Ideally, it is desirable if the input-output characteristic of the VCO is linear as represented by the dotted line; however, in reality, it is generally non-linear as represented by the solid line.
Therefore, conventionally, a method that corrects the modulation sensitivity of the VCO by utilizing a PM-PM table is used. The PM-PM table is used in order to compensate for the nonlinearity of the VCO. The PM-PM table is a table for converting an input voltage (Vtune) of the VCO into the optimum value in order to allow a linear operation for a VCO having a non-linear characteristic. However, even when a PM-PM compensation of the modulation sensitivity of the VCO is conducted, the advantageous effect of the PM-PM compensation is minimized due to variability of the modulation sensitivity of the VCO caused by a temperature fluctuation and the like of the VCO, resulting in the non-linear modulation sensitivity of the VCO.
In order to deal with the above-described problem, conventionally, a circuit that corrects the modulation sensitivity of the VCO by stabilizing the output amplitude Vo of the VCO is disclosed. FIG. 13 is a figure showing one example of a conventional circuit 500 that stabilizes the output amplitude Vo of a VCO 501. In the conventional circuit 500 shown in FIG. 13, an amplitude detector 502 detects the output amplitude Vo of the VCO 501, and outputs a DC voltage corresponding to the detected output amplitude Vo of the VCO 501. An error detection amplifier 503 detects a fluctuation of the output amplitude Vo of the VCO 501 by comparing a reference voltage (Vref) and a DC voltage that corresponds to the output amplitude Vo of the VCO 501. An output signal of the error detection amplifier 503 is inputted to a variable current source 505 via a LPF 504. The variable current source 505 supplies, to the VCO 501, an electric current according to the output signal of the error detection amplifier 503. In this way, the conventional circuit 500 stabilizes the output amplitude Vo of the VCO 501.
FIG. 14A is a figure showing the relationship of an oscillation frequency and a phase noise of the VCO 501. Shown in FIG. 14A are simulation results of a phase noise (With VCO CAL) when the conventional circuit 500 is operated and a phase noise (Without VCO CAL) when the VCO 501 is operated by itself. As shown in FIG. 14A, when compared to an operation of the VCO 501 by itself, an operation of the conventional circuit 500 has a problem where the phase noise detected at a VCO 501 output is deteriorated.
It is known that a noise generated by the amplitude detector 502 is the predominant cause of the phase noise detected at the VCO 501 output. Here, the phase noise detected at the VCO 501 output is calculated by multiplying the noise, generated by the amplitude detector 502, by a closed loop transfer function (low pass function). Therefore, it is possible to improve deterioration of the phase noise by narrowing a loop bandwidth of the conventional circuit 500. FIG. 14A shows an example where the phase noise is improved when the loop bandwidth is shifted from 3 MHz to 1.8 MHz.
Narrowing the loop bandwidth of the conventional circuit 500 can improve deterioration of the phase noise; however, it is also known to deteriorate the response time necessary for the VCO 501 output to settle (i.e., response performance). FIG. 14B is a figure showing a relationship between the loop bandwidth and the response performance. FIG. 14B shows an example where a response characteristic of the VCO 501 output deteriorates when the loop bandwidth is shifted from 3 MHz to 1.8 MHz. Therefore, when the deterioration of the phase noise is improved by narrowing the loop bandwidth of the conventional circuit 500, there is a possibility that the response characteristic of the VCO 501 will not meet the requirement of a system.
Furthermore, the noise generated by the amplitude detector 502 can be reduced by increasing a device size of a transistor which is a component of the amplitude detector 502. However, it is necessary to increase the device size of the transistor substantially in order to reduce the noise to a level that meets the requirement of the system; therefore, there is a limit in reducing the noise by increasing the device size of the transistor. Furthermore, when a transistor with such a large device size is used, a load to the VCO 501 becomes extremely large and a drastic deterioration of a VCO 501 gain is inevitable.
Moreover, a conventional oscillator circuit 510 that stabilizes the output amplitude of the VCO is disclosed in Japanese National Phase PCT Laid-Open Publication No. 2004-527982 (hereinafter, referred to as patent document 1). FIG. 15 is a figure showing a conventional oscillator circuit 510 disclosed in patent document 1. In the conventional oscillator circuit 510 shown in FIG. 15, a plurality of electric current pathways (each pathway having an amplifier 512, a current source 513, and a switch 514) are connect to a LC resonator 511 (VCO) in parallel to each other, and each pathway can be individually turned on/off by the switch 514. An amplitude detector 516 detects an output amplitude of the LC resonator 511. A controller 515 stabilizes the output amplitude of the conventional oscillator circuit 510 by turning on/off a plurality of switches 514 according to an output signal of the amplitude detector 516.
However, if the conventional oscillator circuit 510 shown in FIG. 15 is applied on a system that has a continuous transmission mode, such as an UMTS and the like; when the plurality of switches 514 are switched to adjust the output amplitude, a problem arises where an unwanted radiation is generated in the output signal of the LC resonator 511 due to an operational noise.